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Gravity test confines string theory dimensions

By Stephen Battersby

Gravity has been tested over a shorter distance than ever before. Using a delicate apparatus to measure gravitational forces over just a tenth of a millimetre, a team of physicists has found that they are roughly as Newton’s laws predict.

The result narrows down the possible nature of hidden extra dimensions, which would boost gravity over small scales.

It is extraordinarily difficult to measure gravity over short distances, because weights that are small enough to be manipulated and held so close together only exert very weak gravitational forces.

The most sensitive previous experiment tested the attraction between two masses 0.2 millimetres apart, and found that gravity was no stronger than expected. But what it might be like over smaller scales remained a mystery.

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Tungsten diving board

Now Joshua Long and his colleagues at the University of Colorado, Boulder, have cut that distance in half. Their source of gravity is a metal strip about 20 millimetres long and 0.3 millimetres thick.

“It’s like a tungsten diving board that vibrates up and down”, says Long, now at the Los Alamos National Laboratory in New Mexico. Just 0.1 millimetres below the strip is a second wafer-thin tungsten spring, the “test mass”.

The test mass is tuned to vibrate at the same frequency as the source mass above, so even the faint pull of gravity between the two is enough to set it vibrating in sympathy.

The Colorado group found that the resulting vibrations in the test mass were roughly what you would expect from ordinary gravity, just as Newton would have predicted.

Curled up

This rules out theories that say gravity should be much stronger over such small scales. In string theory, which is physicists’ best attempt so far to unify the forces of nature, space has six or seven extra dimensions – like the familiar three except that they are curled up, perhaps as small as 10-35 metres.

As these dimensions vibrate, they should transmit powerful “modulus forces” over a very short range, which would feel like gravity only tens of thousands of times stronger. The new results show that modulus forces must have a range less than 0.1 millimetres, ruling out versions of the theory that require it to be longer.

The Colorado group are now refining their experiment to measure gravity on smaller scales still. To do this they have removed a gold-plated sapphire shield from between the two tungsten strips, which blocked electromagnetic forces, and replaced it with a thinner beryllium-copper foil, allowing the masses to be moved closer together. They also plan to cool the experiment to cut down on interference from thermal vibrations.